Because of their high lipid and fatty acid content, microalgae, including diatoms, are regarded as a potentially useful source of neutral lipids for use in biodiesel fuel production. Theoretical calculations suggest that an annual oil production of greater than 200 barrels of algal oil per hectare of land may be achievable through mass culture of microalgae such as diatoms, which is 100-fold greater than that of soybeans, a major feedstock currently being used for biodiesel in the USA (Hu et al. The Plant Journal, 54:621-639 (2008)).
Under optimal growth conditions, diatoms and other microalgae synthesize fatty acids primarily for esterification into glycerol-based membrane lipids, which constitute about 5-20% of their dry cell weight. However, under unfavorable environmental conditions, such as during nitrogen deprivation, many algae shift their lipid profile towards the formation and accumulation of neutral lipids, principally in the form of triacylglycerol. Under such unfavorable growth conditions, the total lipid composition of certain microalgae can increase to above 50% of the algae's dry cell weight.
However, in addition to increasing lipid production, culture of microalgae under nutrient deprivation conditions also results in the halt of algal cell division. As a result, the increased lipid content of nutrient starved algae does not lead to an overall increase in lipid productivity. In fact, total rates of lipid production are typically lower under periods of nutrient starvation because higher cellular levels of lipid are offset by crashes in cell division. Thus, using existing technologies, it is possible either to culture microalgae under conditions that promote a high growth rate, or to culture microalgae under conditions that promote an elevated cellular TAG content, but it is not possible to do both simultaneously (Sheehan et al., 1998. A look back at the US Department of Energy's Aquatic Species Program-biodiesal from algae. National Renewable Energy Laboratory, Golden, Colo.; and Yu et al., Journal of Applied Phycology, 21:669-681 (2009)).
The development of microalgae cable of maintaining a lipid-rich phenotype under culture conditions that permit cell division would greatly enhance the economic viability of microalgae, including diatoms, as a source of biofuel precursors. Thus, there is a great need for engineered microalgae with enhanced lipid production during exponential growth.